Fix pre-commit hook for markdownlint

Unfortunately it seems that this hook has been broken since it was
introduced. The problem is that when disabling rules you need an extra
"--" at the end of the list. See: https://github.com/igorshubovych/markdownlint-cli

Frustratingly, passing malformed arguments to markdownlint-cli means
that the hook always passes (!). Upstream issue: igorshubovych/markdownlint-cli#409

I re-ran the hook on this repo, though, and there were no issues with
the markdown.

.pre-commit-config.yaml

@@ -26,5 +26,4 @@ repos:
     rev: v0.35.0
     hooks:
       - id: markdownlint
-        args: ["--disable", "MD013"]
-
+        args: ["--disable", "MD013", "--"]